SECURITY chiefs have today begun their bid to identify an Islamic State militant who beheaded an American journalist, as it was suggested he may have come from London.

More than 500 Britons are believed to have joined IS, whose militants killed journalist James Foley [AP/GETTY]

Prime Minister David Cameron today held emergency meetings with senior intelligence officials following the release of a horrific video showing the killing of James Wright Foley.

The Met Police, who lead Britain's counter-terrorism efforts, have confirmed it is investigating the video.

Scotland Yard said: "The Metropolitan Police Counter Terrorism Command is investigating the contents of the video that was posted online in relation to the alleged murder of James Foley.

"We would like to remind the public that viewing, downloading or disseminating extremist material within the UK may constitute an offence under terrorism legislation."

In the chilling footage, a masked man stands beside Mr Foley as the journalist is forced to read a statement to the camera.

The militant then speaks in what seems to be a British and potentially London accent.

Experts today explained how it is perfectly possible that the militant is another young British extremist, who have been described as the "most vicious and vociferous fighters" within the Islamic State's (IS) ranks.

More than 500 Britons are believed to have joined the al-Qaeda offshoot as they group captured large swathes of Syria and northern Iraq over the past few months.

James Foley went missing in 2012 [EPA]

It's a long trend of brainwashing and being forced into Islamist extremist currents

Charlie Cooper, Quilliam Foundation

Charlie Cooper, from British counter-extremism think-tank the Quilliam Foundation, said there were several steps which could transform a young man from the streets of London to a brutal militant capable of beheading an innocent journalist.

He said: "It's a process which involves a number of different factors. The first is an individual having an identity crisis of some form. He might have faced racist abuse or been kicked out of work or something like that, a crisis which leaves them feeling lost.

"What happens at that point is a charismatic Islamist narrative attracts them and they view it as providing them with answers to the trouble they have faced in their day-to-day life. From that stage they are channelled into an extremist path.

"To get someone from being a non-violent extremist to going to fight in Syria and from then to decapitate an American journalist while they're unarmed is not an immediate process but it is certainly all interlinked.

"Non-violent extremism provides a mood music for jihadism. This guy will have been recruited and bottlenecked into an Islamist extremist ideology which then will have led him to more exposure to propaganda from the Islamic State."

Mr Cooper explained that IS has been so successful at recruiting people from Europe because of its strong social media presence.

He said: "The fact of the matter is that IS propaganda is more effective than any other propaganda. It's better made, it's more in-your-face it's more readily available.

"It would not have been too hard for this man to have had access to this from things like Twitter and Facebook.

"If someone is considering going to fight in somewhere like Syria, it's very easy for them to become recruited to a group like IS.

"All they need to do is go on to Twitter and then follow someone who is very outwardly IS and then direct message them for advice on how to get out there, what airports to go to, how to get across the border form Turkey into Syria, where to meet people, that kind of thing.

"From there they are basically in the hands of IS and they can mould them.

"They are sent to a training camp for both ideology and military training so the combination of those things further indoctrinates them.

"They're an extremist when they leave the UK but they become further radicalised in situ.

"It's a long trend of brainwashing and being forced into Islamist extremist currents which start this kind of thing."

Commenting on the video, Mr Cooper explained that possible reasons behind the decision to use what could be a British person as an executioner could be logistics or extra propaganda power.

Thousands of Iraqis have been displaced, persecuted by IS [AP]

He said: "It's more propaganda value to have someone speaking English because it reaches out to a large number of people.

"There are known to be a number of Americans fighting for IS, not as many as there are from Europe – it's not just Britain but other countries across Europe who have hundreds of people fighting simply because it's more difficult to get from America to Turkey.

"It's simply logistics that will have led to a British person being involved.

"There's also the added bonus of complicating matters between the US and the UK because obviously they would have to deal diplomatically with a British citizen killing an American citizen in a war zone.

"That is going to cause some problems so that is sort of the icing on the cake for the jihadists involved."

Shiraz Maher, from King's College London's International Centre for the Study of Radicalisation, has the involvement of British extremists in Iraq and Syria has been one of "full participation".

He told BBC Radio 4's Today programme: "So we have seen British fighters out there operating as suicide bombers, we have seen them operating as executioners.

"Unfortunately they are amongst some of the most vicious and vociferous fighters who are out there. That is unfortunately just a part of their radicalisation."